I have a global Style Set with a paragraph style in it called "Body." Now I select some paragraphs in a document and use the Paragraph styles palette to assign the style to the selection.

The style is ticked in the palette, as expected. So far, so good. If I then open the current Style Set (Styles > Edit style sets…) and click on the Save button, the paragraphs I previously selected are no longer having the "Body" style, but are instead having a black diamond "◆Body" style, which seems to suggest that a new Document Style has kicked in and that this style is only available in the current document and is not part of the current style set anymore (cf. p. 138 Mellel Guide.) As far as I can see, both styles have the same attributes, so this really does not make any sense to me.

Is this normal? Why are identical Document Styles being created every time I save the current Style Set and why do they override the styles I want to use from the current Style Set?

I always end up with having a LARGE sum of Document Styles, which all seem to have identical attributes to the original paragraph styles in the Style Set.